Do values, desirability, collectability, et cetera differ between sealed sheets that were sold at post offices vs the USPS mail order division?
I doubt it. I wouldn't pay more for sealed panes. But you can check completed sales on eBay
to see if there's a difference. I see that the lot of 10 sealed panes sold for only $35 vs. a face value of $64.
Stamps sold by mail order from the USPS facility in Kansas City also typically come packaged. Currently they include ordinary grey card stock for a stiffener but in the past white cardboard with a USPS logo has been used.